1. Field of the Invention
The present invention relates to a fiber-reinforced composite structural element usable, e.g., for an aircraft fuselage structure and a method of manufacturing the same.
2. Description of the Related Art
A structural element of a fiber-reinforced composite material for use in, e.g., fuselage structures of aircraft, is principally made by superposing on a lay-up jig prepregs composed of reinforcement fibers such as glass fibers previously impregnated with a resin; pressing and curing the prepregs to give a shape in an autoclave, subjecting the thus obtained shape element to, e.g., a secondary adhesion to form a composite sub-assembly, and finally assembling several composite sub-assemblies together by use of fastener means.
FIG. 19 illustrates a conventional stiffener reinforced beam structure as an example. The stiffener reinforced beam structure 100 comprises a beam member 101 previously formed into a C-shape in cross section and having a web portion 101a and flange portions 101b, and stiffeners 102 previously formed into L-shape in cross section and secured to the web portion 101a by means of an adhesive or fasteners. In order to increase the stiffness of the web portion 101a, as shown in FIG. 20, the stiffener reinforced beam structure 100 makes use of means for reinforcing by padding the vicinities of corners R between the flange portions 101b and the web portion 101a.
Japanese Patent Laid-open Pub. No. Hei4-334696 published Nov. 20, 1992, for instance, discloses a reinforcement panel for the fiber-reinforced composite material, which panel comprises a frame having an opening formed by bending a cut portion in the frame to provide flanges, a stringer inserted into the opening of the frame and having an outer surface thereof joined to the flanges, and a skin joined to the frame and the stringer.
Japanese Utility Model Pub. No. Sho63-124119 published Aug. 12, 1988 discloses a coupling for a fiber reinforced composite material which is formed by laying up members composed of fiber-reinforced composite material. The coupling includes a fiber-reinforced composite filler material provided at a boundary part of the coupling. Around or within the filler material is inserted a fiber-reinforced composite material impregnated with a resin and having a lattice-like base in which whiskers such as silicon carbide are dispersed. The members, the filler and the composite material are subjected to a forming/curing treatment.
There are methods to form a box-shaped or similarly shaped structure in which three planes intersect at one point as can be often seen in an aircraft structure. One method is to use a prepreg material of woven fabric having a high shape-maintaining capability such as satin weave. Another method is to use a fastener coupling like metal fittings.
An aircraft structure includes many structural elements having so-called box-like configurations in which three planes intersect at one point. Such structural elements are typically fabricated by machine tool cutting from a metal block or by sheet metal working, but in the case of a composite material by the above-described method.
When producing a composite material component in accordance with a widely used conventional method, prepregs are laid up on a forming jig by hand or by machine (lay-up machine). In the case of producing a composite structure having a complicated configuration, the formation of the composite structure is effected either by manual laying up or by sub-assembling composite components with secondary adhesion. This necessitated an exclusive jig for the adhesion, and also needs repetition of curing under pressure in an autoclave for the purpose of forming the components as well as curing the adhesive because of use of a thermally curing type adhesive from design requirements. This resulted in a relatively high production cost.
It is envisaged that such higher production cost of the composite structure due to these factors is a cause of limited use of composite structural elements in spite of their superior properties in terms of specific strength, specific stiffness, and corrosion resistance. Furthermore, in the case of forming composite material elements having a complicated configuration such as a box-like configuration by means of an integral forming process, use is made of woven fabrics having a high shape-maintaining capability such as satin weave. However, laying up of the prepregs of woven fabric having a high shape-maintaining capability onto the forming jig required high skill to the worker and resulted in a difficulty in thickness control of the composite elements and hence in an unstable quality.